JP2009098560A - Laser scanning optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser scanning optical device, which can correct a relative difference of light quantity for each beam by a simple structure, in multi-beam drawing of an image using at least two semiconductor lasers, while easing the effect of droop characteristic, and consequently suppress image deterioration. <P>SOLUTION: The laser scanning optical device includes a light source part 2A composed of two semiconductor lasers 3a and 3b, 1/2 wavelength plates 4a and 4b, a beam splitter 5, and a collimator lens 6. The 1/2 wavelength plates 4a and 4b correct transmittances of beams α and β to be uniformed, respectively, by adjusting their rotating angles (azimuth angles). The beam splitter 5 has a polarizing film at a joint surface 5a, and transmits/reflects only a predetermined linear polarized light. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser scanning optical device, and more particularly to a laser scanning optical device mounted as a print head in an image forming apparatus such as an electrophotographic copying machine or printer.

  In recent years, in copiers and printers, a multi-beam system that uses a semiconductor laser as a light source and scans the photoreceptor with multiple beams at predetermined intervals in the sub-scanning direction based on the demand for high-speed and high-definition drawing Has become mainstream.

  In the multi-beam method, first, the beams emitted from the respective semiconductor lasers are synthesized in almost the same direction by the synthesis element, and the coupling efficiency difference mainly caused by the variation in the radiation angle of the semiconductor lasers can be adjusted. Necessary. Furthermore, the semiconductor laser has a droop characteristic, which is a cause of image quality degradation. The droop characteristic means that the amount of light of a semiconductor laser tends to decrease as the temperature rises, and when it is driven to light at a constant current, the amount of light decreases slightly due to its own heat generation as the driving time elapses. Image degradation due to droop characteristics appears more significantly in the multi-beam method than in the single-beam method.

With regard to correcting the relative difference in the amount of beam light due to individual differences between semiconductor lasers, Patent Document 1 describes that a transmittance adjusting element is arranged only on one or only one of the beams emitted from two semiconductor lasers. Yes. However, it is difficult to install the transmittance adjusting element for each semiconductor laser in a limited space of the light source unit. Also, if a transmittance adjusting element is provided for only one beam, the amount of light on the photoconductor is lower than that of the other beam. It must be raised and is not practical. Further, Patent Document 1 does not mention any measures for preventing image degradation due to droop characteristics.
JP 2004-45840 A

  Therefore, an object of the present invention is to correct the relative light quantity difference of each beam with a simple configuration and to reduce the influence of droop characteristics when drawing an image with multi-beams using at least two semiconductor lasers. In other words, it is an object of the present invention to provide a laser scanning optical device that can suppress image deterioration.

In order to achieve the above object, the present invention provides:
In a laser scanning optical device that combines beams emitted from at least two semiconductor lasers in substantially the same direction by a combining element, condenses the light by a condensing element, and scans the surface to be scanned in one direction.
A half-wave plate for adjusting the transmittance of the beam is disposed between the semiconductor laser and the combining element,
The composite element has a polarizing film on its bonding surface;
It is characterized by.

  In the laser scanning optical device according to the present invention, by arranging a half-wave plate between the semiconductor laser and the synthesis element, the polarization directions of the respective beams emitted from at least two semiconductor lasers can be substantially equal to each other. The transmittance after the linearly polarized light rotated by 90 ° is transmitted through the combined element having the polarizing film on the bonding surface can be adjusted. The transmittance of each beam can be changed by rotating the half-wave plate about the optical axis, and variations in the output of the semiconductor laser due to the coupling efficiency difference can be corrected. In addition, the absolute value of the coupling efficiency can be changed, and the influence of the droop characteristic can be reduced by increasing the absolute value of the light amount. The semiconductor laser may have one or more light sources.

  In the laser scanning optical apparatus according to the present invention, one of the half-wave plates has a phase angle of about 45 °, and the other has a phase angle of about 0 °. May be adjustable in rotation. The range of adjustment is widened by setting each half-wave plate to a state where the phase angle is high in transmittance. In this case, if the other half-wave plate set so as not to rotate is set to a predetermined rotation angle, the adjustment time is shortened.

  Further, one of the half-wave plates may have a phase angle of about 45 °, and another one may have a phase angle of about 0 °, and both half-wave plates may be adjustable. Alternatively, one of the half-wave plates is initially set to a phase angle of approximately 45 °, and the other one is initially set to a phase angle of approximately 0 °, and either one is rotated from the initial set angle to a predetermined angle. It may be fixed later and the other may be adjusted for rotation. Accordingly, the coupling efficiency difference can be adjusted while keeping the transmittance high.

  It is preferable that the half-wave plate is fixed to a holder having a substantially circular outer periphery, and the holder is set to be rotatable with respect to the optical axis. Further, it is preferable that the beams emitted from the synthesis element have substantially the same elliptical major axis direction.

  2 × N (where N is an integer) semiconductor lasers, one combining element for each of the two semiconductor lasers, and a polarizing film on the bonding surface for the beam emitted from the combining element You may comprise so that it may synthesize | combine in the substantially same direction with the 2nd synthetic | combination element which does not exist. It is possible to adjust the transmittances of beams from four or more light emitting sources.

  Further, the ND filter may be arranged downstream of the combining element in the beam traveling direction. By providing the ND filter, the semiconductor laser can be used at a higher output, and the influence of the droop characteristic can be further alleviated. A holding unit to which a beam intensity measuring sensor is attached and detached may be provided between the ND filter and the combining element. The beam intensity can be measured before the amount of light is reduced by the ND filter, and the influence of measurement errors can be suppressed. Moreover, the measurement workability is improved by providing a holding portion for the measurement sensor.

  Embodiments of a laser scanning optical apparatus according to the present invention will be described below with reference to the accompanying drawings.

(Refer 1st Example and FIGS. 1-7)
FIG. 1 shows a schematic configuration of a laser scanning optical apparatus 1 according to the first embodiment. The apparatus 1 includes a light source unit 2A, a polygon mirror 10 that is rotationally driven at a predetermined speed, scanning lenses 11 and 12 having an fθ function, a horizontal synchronization sensor 14, and a beam condensed on the sensor 14. The condenser lens 15 is housed in the housing 20.

  As shown in FIG. 2, the light source unit 2A includes semiconductor lasers 3a and 3b having a single light source, and a half-wave plate 4a that changes the polarization directions of the beams α and β emitted from the semiconductor lasers 3a and 3b. , 4b and the beams α, β in the same direction, and a synthesizing element (specifically, a beam splitter made of a prism) 5 that aligns the major axis directions of the elliptical shapes of the beams α, β, and the combined beam α , Β is composed of a collimator lens 6, an aperture 7 shown in FIG. 1, and a cylindrical lens 8 for surface tilt correction.

  The beam α emitted from the semiconductor laser 3a is transmitted through the beam splitter 5, and the beam β emitted from the semiconductor laser 3b is reflected by the joint surface 5a of the beam splitter 5, and is synthesized so as to have the same traveling direction. The collimator lens 6 generates parallel light. The operation of the half-wave plates 4a and 4b will be described in detail later. Thereafter, the beams α and β are condensed almost parallel to the sub-scanning direction z by the cylindrical lens 8 and guided to the polygon mirror 10.

  The beams α and β are deflected at a constant angular velocity in the main scanning direction y based on the rotation of the polygon mirror 10, and necessary aberrations are corrected by passing through the scanning lenses 11 and 12, thereby forming an image on the photosensitive drum 25. . The beams α and β are scanned on the photosensitive drum 25 in the main scanning direction y. 1 and 2, the beams α and β are drawn as if they were synthesized on the same optical axis, but in reality they are separated at a predetermined interval in the sub-scanning direction z, and 2 in one scan. Draw a line image. Furthermore, when there are a plurality of light emitting sources of individual semiconductor lasers, a plurality of lines are drawn in one scan.

  Here, the action (combination of the beams α and β) of the half-wave plates 4a and 4b will be described in detail. As shown in FIG. 4, the beam α emitted from the semiconductor laser 3a passes through the half-wave plate 4a having a phase angle set to approximately 0 °, and enters the beam splitter 5 without changing the polarization direction. The beam β emitted from the semiconductor laser 3b passes through the half-wave plate 4b whose phase angle is set to approximately 45 °, and enters the beam splitter 5 as a beam β ′ whose polarization direction is rotated by 90 °.

  In the beam splitter 5, the beams α and β ′ are synthesized in the same direction and in a state where the major axes of the elliptical shapes are aligned. A polarizing film is applied to the joint surface 5a of the beam splitter 5, and only a beam having a predetermined polarization direction is transmitted and reflected.

  5A and 5B show beam transmission / reflection states at the beam splitter 5. FIG. 5A shows a beam incident from the back side of the joint surface 5a, and a beam of lateral vibration indicated by a dotted line is transmitted through almost 100%. However, the longitudinal vibration beam indicated by the solid line transmits only 1% or less. When the vibration angle of the beam gradually changes from the horizontal vibration to the vertical vibration, the transmittance changes from approximately 100% to approximately 0% in accordance with the change in the vibration angle.

  FIG. 5B shows a beam incident toward the bonding surface 5a, and a longitudinal vibration beam indicated by a solid line reflects almost 100%. However, the transverse vibration beam indicated by the dotted line reflects only 1% or less. When the vibration angle of the beam gradually changes from longitudinal vibration to lateral vibration, the transmittance changes from approximately 100% to approximately 0% in accordance with the change in the vibration angle.

  Next, the phase angle of the half-wave plates 4a and 4b will be described. As shown in FIG. 4, the semiconductor laser 3a emits a beam α of lateral vibration. The half-wave plate 4a has a phase angle set to 0 °, and the beam α incident on the half-wave plate 4a is emitted toward the beam splitter 5 with a lateral vibration without substantially reducing the amount of light. The The semiconductor laser 3b emits a transverse vibration beam β. The half-wave plate 4b is set to a phase angle of 45 °, and the beam β incident on the half-wave plate 4b changes to longitudinal vibration (beam β ′) with almost no decrease in the amount of light. It is emitted toward the splitter 5.

  FIG. 6 shows the relationship between the rotation angle (phase angle) of the half-wave plates 4a and 4b and the beam transmittance. The transmittance of the transversely transmitted beam decreases as the phase angle of the half-wave plate 4a increases from 0 °. The transmittance of the longitudinal vibration reflected beam decreases as the phase angle of the half-wave plate 4b decreases from 45 °. In an ideal state where there is no variation in the light amounts of the semiconductor lasers 3a and 3b, the half-wave plate 4a is set to a phase angle of 0 ° for the transmitted beam α, and the half-wave plate 4b is set for the reflected beam β. When the phase angle is set to 45 °, the beams α and β are in a state where there is no difference in light amount, and the maximum light amount (transmittance is almost 100%) can be secured.

  FIG. 7 shows an adjustment example in the case where there is a light amount difference (30%) in the semiconductor lasers 3a and 3b. In this case, if the half-wave plate 4a is set to a phase angle of 16 ° for the transmitted beam α and the half-wave plate 4b is set to a phase angle of 45 ° for the reflected beam β, the beams α and β are It is adjusted so that there is no difference in light quantity. At this time, the transmittance after the beam α of the semiconductor laser 3a having a large amount of light passes through the beam splitter 5 is approximately 70%. Further, in this case, by increasing the outputs of the semiconductor lasers 3a and 3b to increase the respective light amounts, the light amount difference can be eliminated and the influence of the droop characteristic can be reduced.

  As shown in FIG. 3, the half-wave plates 4 a and 4 b are rotatably set around the optical axis in the recess 21 of the housing 20 while being held by the holder 41. The half-wave plates 4 a and 4 b are set to an initial value of 0 ° or 45 ° when the adjusting screw 43 attached to the bracket 42 provided on the side portion of the holder 41 contacts the seat portion 22. ing. In the state where the light source unit 2A alone or the laser scanning optical device 1 incorporating the light source unit 2A is completed, the intensities of the beams α and β are measured, and the adjusting screw 43 is rotated so that the light intensities are uniform. Then, the rotation angle of the half-wave plates 4a and 4b is adjusted to correct the relative difference in light quantity.

  One of the half-wave plates may have a phase angle of about 45 °, and the other one may have a phase angle of about 0 °, and one of the half-wave plates may be rotationally adjustable. That is, if the output (light quantity) of either the transmission side or the reflection side of the semiconductor laser can be discriminated in advance, one of the half-wave plates sets the phase angle to approximately 45 °, and the other one is the phase. The angle may be set to approximately 0 °, and the half-wave plate provided for the semiconductor laser having the larger output may be rotationally adjusted so as to reduce the transmittance. The range of adjustment is widened by setting each half-wave plate to a state where the phase angle is high in transmittance.

  The half-wave plate set so as not to rotate may be set to a predetermined rotation angle. That is, when the variation in the output (light quantity) of the two semiconductor lasers has not been determined in advance, the transmittance after passing through the beam splitter 5 through either the transmission-side or reflection-side half-wave plate decreases. The rotation position is fixed by an angle corresponding to the amount of variation assumed in the direction, and the other half-wave plate is rotated and adjusted so that the light quantity difference between the two is eliminated. If the side that fixes the rotational position is a semiconductor laser with a small amount of light, the reduction in the amount of light after passing through the beam splitter 5 is doubled, but the adjustment man-hours are reduced.

  Further, one of the half-wave plates may have a phase angle of about 45 °, and another one may have a phase angle of about 0 °, and both half-wave plates may be adjustable. That is, when variations in the outputs (light amounts) of the two semiconductor lasers are not determined in advance, the variation in the light amounts is measured by a beam intensity measuring sensor 91 described below after the semiconductor lasers are assembled to the scanning optical device 1. In this case, the transmission-side half-wave plate may be initially set to a phase angle of 0 ° and the reflection-side half-wave plate may be initially set to a phase angle of 45 °, and both half-wave plates may be rotationally adjusted.

  Alternatively, one of the half-wave plates is initially set to a phase angle of approximately 45 °, and the other one is initially set to a phase angle of approximately 0 °, and either one is rotated from the initial set angle to a predetermined angle. It may be fixed later and the other may be adjusted for rotation. In order to eliminate the influence of the droop characteristic of the semiconductor laser as much as possible, it is preferable to use the semiconductor laser at a high output. In this case, the phase angle of the half-wave plate is adjusted so that the transmittance after passing through the beam splitter 5 is reduced, the output variation due to the coupling efficiency difference between the two semiconductor lasers is eliminated, and the absolute light quantity is reduced. What is necessary is just to adjust both. Specifically, after measuring the variation in the amount of light with the beam intensity measuring sensor 91, the half-wave plate of the semiconductor laser having a small beam intensity is rotated and fixed so as to obtain a desired amount of light. The half-wave plate is rotationally adjusted and fixed so that the beam intensity of the semiconductor laser becomes the same as the beam intensity of the previously adjusted semiconductor laser. Thereafter, the absolute light quantity is set high.

(Refer to the second embodiment, FIGS. 8 and 9)
FIG. 8 shows a light source unit 2B of the laser scanning optical apparatus according to the second embodiment. In the light source unit 2B, the ND filter 9 is arranged on the downstream side of the beam traveling direction x of the beam splitter 5. Specifically, the ND filter 9 is disposed on the downstream side of the aperture 7. Other configurations of the laser scanning optical device are the same as those of the first embodiment shown in FIG.

  As is well known, the ND filter 9 has a function of reducing the amount of transmitted light without changing the component of incident light without performing selective absorption. By providing such an ND filter 9, the semiconductor lasers 3a and 3b can be used with higher output, and the influence of the droop characteristic can be further alleviated.

  When measuring the intensity of the beams α and β, the beam intensity measuring sensor 91 (see FIG. 9) is mounted between the ND filter 9 and the aperture 7. A holder 23 detachably attaching the holder 92 of the sensor 91 is provided in a part of the housing 20, and the holder 23 is installed between the ND filter 9 and the aperture 7. When the ND filter 9 is provided, the influence of the measurement error can be suppressed by measuring the beam intensity before the light amount is reduced by the ND filter 9. Further, by providing the housing 20 with the holding portion 23 of the measurement sensor 91, the measurement workability is improved.

  A sensor for measuring the intensity of the beams α and β may be provided immediately after the laser beam emitting portions of the semiconductor lasers 3a and 3b. In this case, the sensor holding portion may be provided in the housing.

(Refer to the third embodiment, FIG. 10)
FIG. 10 shows a light source unit 2C of the laser scanning optical apparatus according to the third embodiment. The light source section 2C is configured by disposing collimator lenses 6a and 6b immediately after the semiconductor lasers 3a and 3b, and the other configuration is the same as that of the first embodiment shown in FIG.

(Refer to the fourth embodiment, FIG. 11)
FIG. 11 shows a light source unit 2D of a laser scanning optical apparatus according to the fourth embodiment. The light source unit 2D includes two semiconductor lasers 3c and 3d, half-wave plates 4c and 4d, and a beam splitter in addition to the light source block including the two semiconductor lasers 3a and 3b, the beam splitter 5, and the collimator lens 6. 5. A light source block including a collimator lens 6 is provided. Furthermore, a beam splitter 50 having no polarizing film on the cementing surface is disposed downstream of each collimator lens 6.

  The configuration of each light source block is the same as that of the first embodiment shown in FIG. A total of four beams emitted from each of the semiconductor lasers 3a to 3d are synthesized in the same direction by the beam splitter 50, irradiated on the photosensitive drum while maintaining a predetermined interval in the sub-scanning direction, and 4 in one scanning. Draw a book line.

  In the fourth embodiment, it can be configured as a laser scanning optical apparatus for drawing with multi-beams provided with 2 × N (where N is an integer) semiconductor lasers.

(Other examples)
The laser scanning optical device according to the present invention is not limited to the above-described embodiments, and can be variously modified within the scope of the gist thereof.

  For example, the image forming apparatus on which the laser scanning optical device is mounted is arbitrary, and may be a monochrome image forming apparatus or a color image forming apparatus, and may be a printer, a copier, a facsimile machine, or a complex machine thereof. Either may be sufficient. Of course, the configuration of the optical system downstream of the polygon mirror is also arbitrary.

It is a top view which shows 1st Example of the laser scanning optical apparatus based on this invention. It is a perspective view which shows the light source part of the said 1st Example. It is an elevation view which shows the attachment state of a half-wave plate. It is a perspective view explaining the effect | action of a 1/2 wavelength plate and a beam synthesizing element. It is a perspective view which shows the permeation | transmission / reflection state of the beam in a beam synthetic | combination element. It is a graph which shows the change of the beam transmittance with respect to the phase angle of a half-wave plate. It is the same graph as FIG. 6, and the example of adjustment is shown. It is a top view which shows the light source part of 2nd Example of the laser scanning optical apparatus based on this invention. It is a perspective view which shows the sensor for a measurement in the said 2nd Example, and its holding | maintenance part. It is a perspective view which shows the light source part of 3rd Example of the laser scanning optical apparatus based on this invention. It is a top view which shows the light source part of 4th Example of the laser scanning optical apparatus based on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Laser scanning optical apparatus 2A, 2B, 2C, 2D ... Light source part 3a, 3b, 3c, 3d ... Semiconductor laser 4a, 4b, 4c, 4d ... 1/2 wavelength plate 5,50 ... Beam splitter 5a ... Joint surface 6 6a, 6b ... Collimator lens 9 ... ND filter 20 ... Housing 23 ... Holding part 41 ... Holder 91 ... Sensor for measuring beam intensity α, β, β '... Beam

Claims (10)

In a laser scanning optical device that combines beams emitted from at least two semiconductor lasers in substantially the same direction by a combining element, condenses the light by a condensing element, and scans the surface to be scanned in one direction.
A half-wave plate for adjusting the transmittance of the beam is disposed between the semiconductor laser and the combining element,
The composite element has a polarizing film on its bonding surface;
A laser scanning optical device.   One of the half-wave plates has a phase angle of about 45 °, and the other one has a phase angle of about 0 °, and either one of the half-wave plates can be rotated. The laser scanning optical apparatus according to claim 1, wherein   3. The laser scanning optical apparatus according to claim 2, wherein the other half-wave plate set so as not to rotate is set at a predetermined rotation angle.   One of the half-wave plates has a phase angle of about 45 °, and the other one has a phase angle set to about 0 °, and both half-wave plates can be rotationally adjusted. The laser scanning optical apparatus according to claim 1.   One of the half-wave plates is initially set to a phase angle of approximately 45 °, and the other one is initially set to a phase angle of approximately 0 °, and after either one is rotated from the initial set angle to a predetermined angle, The laser scanning optical device according to claim 1, wherein the laser scanning optical device is fixed and the other is rotationally adjusted.   6. The half wave plate is fixed to a holder having a substantially circular outer periphery, and the holder is set to be rotatable with respect to the optical axis. A laser scanning optical device according to claim 1.   7. The laser scanning optical apparatus according to claim 1, wherein the beam emitted from the combining element has an elliptical long axis direction substantially coincident.   2 × N (where N is an integer) semiconductor lasers, one synthesis element for each of the two semiconductor lasers, and a polarizing film on the bonding surface for the beam emitted from the synthesis element 8. The laser scanning optical apparatus according to claim 1, wherein the second combining elements are combined in substantially the same direction.   9. The laser scanning optical apparatus according to claim 1, wherein an ND filter is disposed downstream of the combining element in a beam traveling direction.   The laser scanning optical apparatus according to claim 9, further comprising a holding unit to which a beam intensity measuring sensor is attached and detached between the combining element and the ND filter.

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